Thermoplastic foam products can be produced by a wide variety of processes, of which extrusion is but one, that are in part responsible for the wide variety of foam products available today. Foams range in consistency from rigid materials suitable for structural use to flexible substances for soft cushions and packaging materials. These foams range in cellular formation from open or interconnecting-cell foams to closed or unicell foams. The cell structure may range from large to fine. Electrical, thermal, mechanical, and chemical properties can be varied within wide limits depending on the thermoplastic resin composition and the method chosen to create the foam. Foamed thermoplastics range in density anywhere from about 10 kg/m.sup.3 to over 1,000 kg/m.sup.3, although the latter perhaps more properly are called microcellular structures. True foams are considered to have a density of less than about 800 kg/M.sup.3.
Many methods have been developed for the manufacture of foamed thermoplastics, which generally can be classified into three groups: 1) methods for adding a gaseous "blowing agent" to the thermoplastic mass during processing, 2) methods for producing a gaseous blowing agent in the thermoplastic mass during processing, and 3) methods for forming a thermoplastic mass from granules to obtain a cellular structure. Similar blowing agents sometimes are used in the various methods to produce foams. However, the effectiveness of a particular blowing agent varies considerably depending on the thermoplastic resin composition, the method chosen, the process conditions, the additives used, and the product sought.
Blowing agents work by expanding a thermoplastic resin to produce a cellular thermoplastic structure having far less density than the resin from which the foam is made. Bubbles of gas form around "nucleation sites" and are expanded by heat or reduced pressure or by a process of chemical reaction in which a gas is evolved. A nucleation site is a small particle or conglomerate of small particles that promotes the formation of a gas bubble in the resin. Additives may be incorporated into the resin to promote nucleation for a particular blowing agent and, consequently, a more uniform pore distribution. However, the foam is maintained by replacing the blowing agent in the cells with air. Diffusivity of the blowing agent out of the cells relative to air coming into the cells impacts the stability of the foam over time and whether the cells of the foam may collapse. Additives may be incorporated into the resin and process conditions may be adjusted to assist in controlling the diffusivity of the blowing agent, to promote foam stability, and to limit collapse of the foam to acceptable limits.
Methods for producing a blowing agent in situ usually involve a chemical reaction that evolves gas. Polyethylene, silicone, epoxy, and vinyl foams have all been produced by adding a substance that will produce a gaseous blowing agent chemically. For example, dinitroso compounds and hydrazides, which evolve nitrogen gas on decomposition, and bicarbonates, which evolve carbon dioxide, have been added to thermoplastic resins to produce foams.
Polystyrene foams often are produced by "bead molding," in which partially expanded granules or beads are heated in a mold in the presence of a blowing agent to expand and fuse the particles into a rigid unicellular structure. A volatile organic compound or some other gaseous blowing agent is impregnated into the beads. Heat is applied and the pressure is released to cause the beads to expand and fuse.
There are several methods for adding a blowing agent to a thermoplastic resin during processing to produce a foam. Ureaformaldehyde and polyvinylformaldehyde foams have been produced by whipping air into a heated thermoplastic mass before it sets. Polyolefinic foams have been produced by introducing air or some other gas or volatile solvent into a heated thermoplastic polyolefin mass and then heating the mass or reducing pressure to expand the gas and form pores of a desirable size. One more specific method is to impregnate a thermoplastic resin with blowing agent under heat and pressure in a closed vessel. The pressure is released to expand the blowing agent to form "prefoamed," or partially expanded, beads. Prefoamed beads usually are further expanded in an enclosed vessel such as a mold to produce a molded foam product, as is discussed above.
As examples of the use of various blowing agents for molding and prefoamed bead production, Kloker et al. U.S. Pat. No. 4,120,923 and Yoshimura et al. U.S. Pat. No. 4,464,484 disclose the use of the inert gas carbon dioxide as a blowing agent for molded polyolefin foam articles and for polymer beads, respectively. Yoshimura et al. U.S. Pat. No. 4,464,484 disclose that a mixture of carbon dioxide and aliphatic hydrocarbons and halogenated aliphatic hydrocarbons, including CFCs and HCFCs, are useful blowing agents for polyolefin beads. Broad ranges of blends of VOCs, CFCs, and HCFCs with carbon dioxide are disclosed and mixtures of butane or dichlorodifluoromethane and carbon dioxide are exemplified.
Another more specific method, to which the invention claimed herein relates, is to mix the blowing agent with molten thermoplastic resin under pressure and then extrude the mixture through a forming die into a zone of reduced pressure. Shaped extruded foams can be produced by this method using a forming die of particular configuration. Plank, which can be cut to a desirable shape, and thin foam sheets are produced in this manner.
Many of the halogenated hydrocarbons have been used for several years as blowing agents in various methods for producing extruded foams from thermoplastic resins. The halogenated hydrocarbons include the chlorofluorocarbons ("CFCs") and hydrochlorofluorocarbons ("HCFCs"). CFCs and HCFCs are readily impregnable in thermoplastic resins and are readily expandable under relatively mild conditions. CFCs and HCFCs generally produce foams of high quality with a minimum of processing difficulty. The pore size is controllable, the foam has good stability with minimum tendency to collapse after a period of time, and the surface characteristics of the foam are smooth and desirable. Also, CFCs, HCFCs, and other halogenated hydrocarbons typically are either not flammable or are of low flammability, which greatly reduces the care with which they may be used. These compounds have the further advantage of low toxicity. However, governmental regulation is phasing out use of halogenated hydrocarbons because the halogenated hydrocarbons may be responsible for damage to the earth's ozone layer.
Producers of thermoplastic foam products have been seeking alternatives to CFC and HCFC blowing agents for a number of years to reduce or eliminate altogether the amount of halogenated hydrocarbons used. A number of volatile organic compounds (VOCs) have been proposed, although many of these also are somewhat objectionable. VOCs include the light aliphatic hydrocarbons such as ethane, propane, n-butane, isobutane, butylene, isobutene, pentane, neopentane, and hexane, to name but a few. The diffusivity of VOCs can be many times faster than that of the halogenated hydrocarbons and harder to control. Foam collapse and stability problems have been encountered, although high quality foams have been produced using, for example, butane.
VOCs typically are volatile and flammable, thus presenting handling problems and safety concerns. For example, Robin et al. U.S. Pat. No. 5,314,926 describes a blowing agent comprising a mixture of one or more hydrocarbons or partially halogenated alkanes with a fluorinated propane, 1,1,1,2,3,3,3-heptafluoropropane. The hydrocarbons are said to include propane, butane, isobutane, n-pentane, i-pentane, neopentane, n-hexane, 2 methylpentane, 3-methylpentane, and 2,2-dimethylbuntane. The fluorinated propane is said to be useful even in small amounts in reducing the flammability of foamable plastics including polystyrene, polyvinyl chloride, polyethylene, and other non-polyisocyanate based foams.
The behavior of VOCs in various thermoplastic resins and the foams prepared therefrom is somewhat unpredictable due to the differences in volatility of the various VOCs and VOC blends, the differences in the foaming behavior of different thermoplastic resins, the wide variation in kinds and amounts of processing additives that are added to the different thermoplastic resins, and a host of other factors too numerous to mention here.
As an example of extrusion foaming, Watanabe et al. U.S. Pat. No. 4,214,054 describe numerous volatile organic blowing agents including various CFCs, VOCs, and the use of decomposable gas-releasing chemical blowing agents for producing extruded polyolefin foams from particular resin compositions.
Johnson U.S. Pat. No. 3,966,373 proposes a method and apparatus for making relatively dense structural foam profiles having a foam core and a dense skin. A partially expanded extruded thermoplastic polymer composition is conveyed through a chilled shaping passage moving at the same rate as the foam to eliminate friction. The dense skin is formed by the chilled passage while the polymer resin is still expanding. Foaming agents are said to include nitrogen, carbon dioxide, lower molecular weight paraffins such as propane, butane, and methylchloride, lower molecular weight olefins such as ethylene, propylene, and butylene or mixtures of the above. No specific mixtures of foaming agents are disclosed. A preferred thermoplastic composition for extrusion to form tongue depressors or ice cream sticks of 320 to 1000 kilograms per cubic meter is disclosed to include polystyrene beads having a pentane blowing agent integrated therewith.
Gilbert U.S. Pat. No. 3,488,746 discloses a process for preparing a foamed polyethylene layflat tube by blow extruding a foamable polyethylene resin composition through an annular die. Blowing agents are said to include lower aliphatic hydrocarbons such as ethane, propane, butane, or pentane, lower alkyl halides such as methylchloride, trichloromethane, or 1,2-dichlorotetrafluorethane and inorganic gases such as carbon dioxide or nitrogen. Butane and isobutylene are said to be preferred. Highly active nucleating agents such as silica or alumina or small quantities of decomposable nucleating agents are disclosed in quantities of up to about 5 percent by weight of the resin.
Vesilyn U.S. Pat. No. 3,287,477 discloses extrusion apparatus and methods for preparing polymer foams and is primarily directed to polystyrene foam. Vesilyn discloses that the extrusion apparatus can also be operated with other polymers including polymers derived from ethylene monomers to produce foam sheets. Blowing agents are said to include examples such as methane, ethane, propane, butane, n-pentane, isopentane, neopentane, hexanes, heptanes, and a variety of others.
Vonken et al. U.S. Pat. No. 5,484,649 describes extruded polystyrene expanded films and shaped articles, including building materials, that have been prepared using propane, butane, or mixtures of propane and butane as blowing agents in polystyrene melts that are treated with flameproofing agents and nucleating agents. The foams are said preferably to rest for at least about one week to adjust the amount of residual blowing agent to the minimum required for further expansion by heat treatment. The post-extrusion expansion step is said to increase the thickness of the extrudate by a factor of from about 1.8 to 2.
Polystyrene typically is considered to have different foaming characteristics from the polyolefins. Polystyrene is an amorphous material whereas the polyolefins are semi-crystalline.
Butane blowing agents normally are used for foam extrusion of low density polyolefin foams (over 30% expansion) in an amount of from about 15% to 16% by weight of the resin, typically in combination with active nucleating agents in amounts of about 0.5% by weight of the resin. Butane is extremely flammable and at the levels used for blowing agent an aging period typically is provided to reduce residual butane blowing agents below explosive limits prior to shipment of the foam.
Propane has successfully been used as a blowing agent for producing polyethylene thin foam sheet and foams of about 11 mm thickness or less, despite its volatility and flammability. However, there has been no disclosure of or suggestion to use propane as a blowing agent to produce thicker grade foams or plank and there has been no disclosure or suggestion of improvements in properties of such foams.
It is desirable to continue to develop combinations of blowing agents and thermoplastic resins that can result in foam products having improved properties, including safety, foam physical characteristics, and other properties.